1. Field of the Invention
The present invention relates to noise detection and location and, more particularly, to a small array of microphones and associated processing system that can be used to locate intermittent noise/sound sources.
2. Description of the Related Art
Industrial noise is a vexing environmental problem, affecting the workplace as well as public spaces. One of the critical steps to better engineering, through active flow control or other relevant design changes, is the localization of the sources, which can be small, intermittent and jittery. These sources can be a problem in high speed transportation (wind noise in automotive industry) to wind turbine acoustic impact, from machinery noise to household appliances, and from jet noise to many forms of acoustic tomography and related non-invasive diagnostics.
Parabolic antennae are used to focus the sound propagating along one straight line parallel to the parabola's axis into its focal point (single microphone). These systems were known for eavesdropping opportunities near stone or frescoed cupolas in ancient times, and are displayed on the sidelines of modern sports fields. Their electromagnetic siblings are ubiquitous as satellite dishes. However, they are incapable of providing distance information.
Acoustic antennae are combinations of microphones collecting sound from assorted sources, and they provide a measure of spatial localization. Related technology is very efficient in the context of phased-array radars, but the difference in wavelength-dependent resolution is critical at the application level. The signals pi(t), i=1 . . . N (where N is the number of microphones) are collected simultaneously and combined, either in real time or in post-processing mode. When a target is specified, it is a simple matter of Cartesian geometry to calculate the distances Di between the known microphone locations and the target; these distances are then converted to propagation times Ti=Di/c for a given ambient speed of sound c. Knowing that acoustic fluctuations from the targeted source reach the microphones at different times, the relative lags (differences in arrival times) tij=Ti−Tj, can be used to synchronize the signals to be in phase with each other. The microphones are typically arranged as a plane non-periodic array, e.g., along the arms of logarithmic spirals.
Existing systems for noise location are based on phased-array antennae and beam forming algorithms. The antennae are (typically flat) arrays of several dozen microphones arranged in irregular patterns. Recording simultaneously from all microphones yields the raw data. The ‘focusing’ on a given target is achieved by introducing a relative delay between the signals to account for different propagation times from source to microphone; averaging these lagged signals amounts to seeking constructive interference from the target area and partially-cancelling interference for all mismatched lags. The spatial resolution of these antennae is of the order of one wavelength of the acoustic wave—for 1 kHz sound, the ‘ball’ of localization is over one foot in diameter, whereas possible responsible flow patterns are in the inch range or smaller. So, existing antennae cannot resolve the spatial intermittency and random motion of many source.